According to scientists
from Agroscope – a state-run center for agricultural research in
Switzerland – the holes found in cheeses such as Emmental and
Appenzell are caused by tiny bits of hay present in the milk used
to make the products.

In a series of tests, researchers added different amounts of hay
dust to the milk and discovered that it allowed them to control
the number of holes present.

The findings also explain why the holes have been declining over
the last 15 years – because the modern technique of milking a cow
with a sealed machine has eliminated the possibility of hay
creeping into the milk.

Simply put, it's the “disappearance of the traditional
bucket” used during milking that caused the difference,
Agroscope spokesman Regis Nyffeler said, as quoted by AFP.

The milking machines “completely did away with the presence
of tiny hay particles in the milk,” he added.

Agroscope said the issue had been under study since at least
1917, when American William Clark published a detailed study and
came to the conclusion that it was caused by carbon dioxide
released by bacteria present in the milk.

A less scientific – but widely circulated – explanation was that
the holes were caused by cheese-loving mice.